mm: enable page walking API to lock vmas during the walk

walk_page_range() and friends often operate under write-locked mmap_lock.
With introduction of vma locks, the vmas have to be locked as well during
such walks to prevent concurrent page faults in these areas.  Add an
additional member to mm_walk_ops to indicate locking requirements for the
walk.

The change ensures that page walks which prevent concurrent page faults
by write-locking mmap_lock, operate correctly after introduction of
per-vma locks.  With per-vma locks page faults can be handled under vma
lock without taking mmap_lock at all, so write locking mmap_lock would
not stop them.  The change ensures vmas are properly locked during such
walks.

A sample issue this solves is do_mbind() performing queue_pages_range()
to queue pages for migration.  Without this change a concurrent page
can be faulted into the area and be left out of migration.

Link: https://lkml.kernel.org/r/20230804152724.3090321-2-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Suggested-by: Jann Horn <jannh@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michel Lespinasse <michel@lespinasse.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/arch/powerpc/mm/book3s64/subpage_prot.c b/arch/powerpc/mm/book3s64/subpage_prot.c
index 0dc8555..ec98e52 100644 (file)
--- a/arch/powerpc/mm/book3s64/subpage_prot.c
+++ b/arch/powerpc/mm/book3s64/subpage_prot.c
@@ -145,6 +145,7 @@ static int subpage_walk_pmd_entry(pmd_t *pmd, unsigned long addr,
 
 static const struct mm_walk_ops subpage_walk_ops = {
        .pmd_entry      = subpage_walk_pmd_entry,
+       .walk_lock      = PGWALK_WRLOCK_VERIFY,
 };
 
 static void subpage_mark_vma_nohuge(struct mm_struct *mm, unsigned long addr,

diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
index ea3d61d..161d0b3 100644 (file)
--- a/arch/riscv/mm/pageattr.c
+++ b/arch/riscv/mm/pageattr.c
@@ -102,6 +102,7 @@ static const struct mm_walk_ops pageattr_ops = {
        .pmd_entry = pageattr_pmd_entry,
        .pte_entry = pageattr_pte_entry,
        .pte_hole = pageattr_pte_hole,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask,

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 9c8af31..906a7bf 100644 (file)
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2514,6 +2514,7 @@ static int thp_split_walk_pmd_entry(pmd_t *pmd, unsigned long addr,
 
 static const struct mm_walk_ops thp_split_walk_ops = {
        .pmd_entry      = thp_split_walk_pmd_entry,
+       .walk_lock      = PGWALK_WRLOCK_VERIFY,
 };
 
 static inline void thp_split_mm(struct mm_struct *mm)
@@ -2565,6 +2566,7 @@ static int __zap_zero_pages(pmd_t *pmd, unsigned long start,
 
 static const struct mm_walk_ops zap_zero_walk_ops = {
        .pmd_entry      = __zap_zero_pages,
+       .walk_lock      = PGWALK_WRLOCK,
 };
 
 /*
@@ -2655,6 +2657,7 @@ static const struct mm_walk_ops enable_skey_walk_ops = {
        .hugetlb_entry          = __s390_enable_skey_hugetlb,
        .pte_entry              = __s390_enable_skey_pte,
        .pmd_entry              = __s390_enable_skey_pmd,
+       .walk_lock              = PGWALK_WRLOCK,
 };
 
 int s390_enable_skey(void)
@@ -2692,6 +2695,7 @@ static int __s390_reset_cmma(pte_t *pte, unsigned long addr,
 
 static const struct mm_walk_ops reset_cmma_walk_ops = {
        .pte_entry              = __s390_reset_cmma,
+       .walk_lock              = PGWALK_WRLOCK,
 };
 
 void s390_reset_cmma(struct mm_struct *mm)
@@ -2728,6 +2732,7 @@ static int s390_gather_pages(pte_t *ptep, unsigned long addr,
 
 static const struct mm_walk_ops gather_pages_ops = {
        .pte_entry = s390_gather_pages,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 /*

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index fc74496..fafff1b 100644 (file)
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -757,12 +757,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
 static const struct mm_walk_ops smaps_walk_ops = {
        .pmd_entry              = smaps_pte_range,
        .hugetlb_entry          = smaps_hugetlb_range,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 static const struct mm_walk_ops smaps_shmem_walk_ops = {
        .pmd_entry              = smaps_pte_range,
        .hugetlb_entry          = smaps_hugetlb_range,
        .pte_hole               = smaps_pte_hole,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 /*
@@ -1244,6 +1246,7 @@ static int clear_refs_test_walk(unsigned long start, unsigned long end,
 static const struct mm_walk_ops clear_refs_walk_ops = {
        .pmd_entry              = clear_refs_pte_range,
        .test_walk              = clear_refs_test_walk,
+       .walk_lock              = PGWALK_WRLOCK,
 };
 
 static ssize_t clear_refs_write(struct file *file, const char __user *buf,
@@ -1621,6 +1624,7 @@ static const struct mm_walk_ops pagemap_ops = {
        .pmd_entry      = pagemap_pmd_range,
        .pte_hole       = pagemap_pte_hole,
        .hugetlb_entry  = pagemap_hugetlb_range,
+       .walk_lock      = PGWALK_RDLOCK,
 };
 
 /*
@@ -1934,6 +1938,7 @@ static int gather_hugetlb_stats(pte_t *pte, unsigned long hmask,
 static const struct mm_walk_ops show_numa_ops = {
        .hugetlb_entry = gather_hugetlb_stats,
        .pmd_entry = gather_pte_stats,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 /*

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 27a6df4..27cd1e5 100644 (file)
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -6,6 +6,16 @@
 
 struct mm_walk;
 
+/* Locking requirement during a page walk. */
+enum page_walk_lock {
+       /* mmap_lock should be locked for read to stabilize the vma tree */
+       PGWALK_RDLOCK = 0,
+       /* vma will be write-locked during the walk */
+       PGWALK_WRLOCK = 1,
+       /* vma is expected to be already write-locked during the walk */
+       PGWALK_WRLOCK_VERIFY = 2,
+};
+
 /**
  * struct mm_walk_ops - callbacks for walk_page_range
  * @pgd_entry:         if set, called for each non-empty PGD (top-level) entry
@@ -66,6 +76,7 @@ struct mm_walk_ops {
        int (*pre_vma)(unsigned long start, unsigned long end,
                       struct mm_walk *walk);
        void (*post_vma)(struct mm_walk *walk);
+       enum page_walk_lock walk_lock;
 };
 
 /*

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 2fcc973..e0e59d4 100644 (file)
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -386,6 +386,7 @@ out:
 static const struct mm_walk_ops damon_mkold_ops = {
        .pmd_entry = damon_mkold_pmd_entry,
        .hugetlb_entry = damon_mkold_hugetlb_entry,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 static void damon_va_mkold(struct mm_struct *mm, unsigned long addr)
@@ -525,6 +526,7 @@ out:
 static const struct mm_walk_ops damon_young_ops = {
        .pmd_entry = damon_young_pmd_entry,
        .hugetlb_entry = damon_young_hugetlb_entry,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 static bool damon_va_young(struct mm_struct *mm, unsigned long addr,

diff --git a/mm/hmm.c b/mm/hmm.c
index 855e25e..277ddca 100644 (file)
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -562,6 +562,7 @@ static const struct mm_walk_ops hmm_walk_ops = {
        .pte_hole       = hmm_vma_walk_hole,
        .hugetlb_entry  = hmm_vma_walk_hugetlb_entry,
        .test_walk      = hmm_vma_walk_test,
+       .walk_lock      = PGWALK_RDLOCK,
 };
 
 /**

diff --git a/mm/ksm.c b/mm/ksm.c
index d20d766..d7b5b95 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -455,6 +455,12 @@ static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long nex
 
 static const struct mm_walk_ops break_ksm_ops = {
        .pmd_entry = break_ksm_pmd_entry,
+       .walk_lock = PGWALK_RDLOCK,
+};
+
+static const struct mm_walk_ops break_ksm_lock_vma_ops = {
+       .pmd_entry = break_ksm_pmd_entry,
+       .walk_lock = PGWALK_WRLOCK,
 };
 
 /*
@@ -470,16 +476,17 @@ static const struct mm_walk_ops break_ksm_ops = {
  * of the process that owns 'vma'.  We also do not want to enforce
  * protection keys here anyway.
  */
-static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
+static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
 {
        vm_fault_t ret = 0;
+       const struct mm_walk_ops *ops = lock_vma ?
+                               &break_ksm_lock_vma_ops : &break_ksm_ops;
 
        do {
                int ksm_page;
 
                cond_resched();
-               ksm_page = walk_page_range_vma(vma, addr, addr + 1,
-                                              &break_ksm_ops, NULL);
+               ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
                if (WARN_ON_ONCE(ksm_page < 0))
                        return ksm_page;
                if (!ksm_page)
@@ -565,7 +572,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
        mmap_read_lock(mm);
        vma = find_mergeable_vma(mm, addr);
        if (vma)
-               break_ksm(vma, addr);
+               break_ksm(vma, addr, false);
        mmap_read_unlock(mm);
 }
 
@@ -871,7 +878,7 @@ static void remove_trailing_rmap_items(struct ksm_rmap_item **rmap_list)
  * in cmp_and_merge_page on one of the rmap_items we would be removing.
  */
 static int unmerge_ksm_pages(struct vm_area_struct *vma,
-                            unsigned long start, unsigned long end)
+                            unsigned long start, unsigned long end, bool lock_vma)
 {
        unsigned long addr;
        int err = 0;
@@ -882,7 +889,7 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma,
                if (signal_pending(current))
                        err = -ERESTARTSYS;
                else
-                       err = break_ksm(vma, addr);
+                       err = break_ksm(vma, addr, lock_vma);
        }
        return err;
 }
@@ -1029,7 +1036,7 @@ static int unmerge_and_remove_all_rmap_items(void)
                        if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
                                continue;
                        err = unmerge_ksm_pages(vma,
-                                               vma->vm_start, vma->vm_end);
+                                               vma->vm_start, vma->vm_end, false);
                        if (err)
                                goto error;
                }
@@ -2530,7 +2537,7 @@ static int __ksm_del_vma(struct vm_area_struct *vma)
                return 0;
 
        if (vma->anon_vma) {
-               err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
+               err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end, true);
                if (err)
                        return err;
        }
@@ -2668,7 +2675,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
                        return 0;               /* just ignore the advice */
 
                if (vma->anon_vma) {
-                       err = unmerge_ksm_pages(vma, start, end);
+                       err = unmerge_ksm_pages(vma, start, end, true);
                        if (err)
                                return err;
                }

diff --git a/mm/madvise.c b/mm/madvise.c
index 886f060..bfe0e06 100644 (file)
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -233,6 +233,7 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,
 
 static const struct mm_walk_ops swapin_walk_ops = {
        .pmd_entry              = swapin_walk_pmd_entry,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 static void shmem_swapin_range(struct vm_area_struct *vma,
@@ -534,6 +535,7 @@ regular_folio:
 
 static const struct mm_walk_ops cold_walk_ops = {
        .pmd_entry = madvise_cold_or_pageout_pte_range,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 static void madvise_cold_page_range(struct mmu_gather *tlb,
@@ -757,6 +759,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 
 static const struct mm_walk_ops madvise_free_walk_ops = {
        .pmd_entry              = madvise_free_pte_range,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 static int madvise_free_single_vma(struct vm_area_struct *vma,

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e8ca4bd..315fd5f 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6024,6 +6024,7 @@ static int mem_cgroup_count_precharge_pte_range(pmd_t *pmd,
 
 static const struct mm_walk_ops precharge_walk_ops = {
        .pmd_entry      = mem_cgroup_count_precharge_pte_range,
+       .walk_lock      = PGWALK_RDLOCK,
 };
 
 static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
@@ -6303,6 +6304,7 @@ put:                      /* get_mctgt_type() gets & locks the page */
 
 static const struct mm_walk_ops charge_walk_ops = {
        .pmd_entry      = mem_cgroup_move_charge_pte_range,
+       .walk_lock      = PGWALK_RDLOCK,
 };
 
 static void mem_cgroup_move_charge(void)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 9a28503..139b31f 100644 (file)
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -831,6 +831,7 @@ static int hwpoison_hugetlb_range(pte_t *ptep, unsigned long hmask,
 static const struct mm_walk_ops hwp_walk_ops = {
        .pmd_entry = hwpoison_pte_range,
        .hugetlb_entry = hwpoison_hugetlb_range,
+       .walk_lock = PGWALK_RDLOCK,
 };
 
 /*

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index c53f8be..ec2eace 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -718,6 +718,14 @@ static const struct mm_walk_ops queue_pages_walk_ops = {
        .hugetlb_entry          = queue_folios_hugetlb,
        .pmd_entry              = queue_folios_pte_range,
        .test_walk              = queue_pages_test_walk,
+       .walk_lock              = PGWALK_RDLOCK,
+};
+
+static const struct mm_walk_ops queue_pages_lock_vma_walk_ops = {
+       .hugetlb_entry          = queue_folios_hugetlb,
+       .pmd_entry              = queue_folios_pte_range,
+       .test_walk              = queue_pages_test_walk,
+       .walk_lock              = PGWALK_WRLOCK,
 };
 
 /*
@@ -738,7 +746,7 @@ static const struct mm_walk_ops queue_pages_walk_ops = {
 static int
 queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
                nodemask_t *nodes, unsigned long flags,
-               struct list_head *pagelist)
+               struct list_head *pagelist, bool lock_vma)
 {
        int err;
        struct queue_pages qp = {
@@ -749,8 +757,10 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
                .end = end,
                .first = NULL,
        };
+       const struct mm_walk_ops *ops = lock_vma ?
+                       &queue_pages_lock_vma_walk_ops : &queue_pages_walk_ops;
 
-       err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
+       err = walk_page_range(mm, start, end, ops, &qp);
 
        if (!qp.first)
                /* whole range in hole */
@@ -1078,7 +1088,7 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
        vma = find_vma(mm, 0);
        VM_BUG_ON(!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)));
        queue_pages_range(mm, vma->vm_start, mm->task_size, &nmask,
-                       flags | MPOL_MF_DISCONTIG_OK, &pagelist);
+                       flags | MPOL_MF_DISCONTIG_OK, &pagelist, false);
 
        if (!list_empty(&pagelist)) {
                err = migrate_pages(&pagelist, alloc_migration_target, NULL,
@@ -1321,12 +1331,8 @@ static long do_mbind(unsigned long start, unsigned long len,
         * Lock the VMAs before scanning for pages to migrate, to ensure we don't
         * miss a concurrently inserted page.
         */
-       vma_iter_init(&vmi, mm, start);
-       for_each_vma_range(vmi, vma, end)
-               vma_start_write(vma);
-
        ret = queue_pages_range(mm, start, end, nmask,
-                         flags | MPOL_MF_INVERT, &pagelist);
+                         flags | MPOL_MF_INVERT, &pagelist, true);
 
        if (ret < 0) {
                err = ret;

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 8365158..d5f4923 100644 (file)
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -279,6 +279,7 @@ next:
 static const struct mm_walk_ops migrate_vma_walk_ops = {
        .pmd_entry              = migrate_vma_collect_pmd,
        .pte_hole               = migrate_vma_collect_hole,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 /*

diff --git a/mm/mincore.c b/mm/mincore.c
index b7f7a51..dad3622 100644 (file)
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -176,6 +176,7 @@ static const struct mm_walk_ops mincore_walk_ops = {
        .pmd_entry              = mincore_pte_range,
        .pte_hole               = mincore_unmapped_range,
        .hugetlb_entry          = mincore_hugetlb,
+       .walk_lock              = PGWALK_RDLOCK,
 };
 
 /*

diff --git a/mm/mlock.c b/mm/mlock.c
index 0a0c996..479e09d 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -371,6 +371,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
 {
        static const struct mm_walk_ops mlock_walk_ops = {
                .pmd_entry = mlock_pte_range,
+               .walk_lock = PGWALK_WRLOCK_VERIFY,
        };
 
        /*

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 6f658d4..3aef134 100644 (file)
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -568,6 +568,7 @@ static const struct mm_walk_ops prot_none_walk_ops = {
        .pte_entry              = prot_none_pte_entry,
        .hugetlb_entry          = prot_none_hugetlb_entry,
        .test_walk              = prot_none_test,
+       .walk_lock              = PGWALK_WRLOCK,
 };
 
 int

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 2022333..9b2d23f 100644 (file)
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -400,6 +400,33 @@ static int __walk_page_range(unsigned long start, unsigned long end,
        return err;
 }
 
+static inline void process_mm_walk_lock(struct mm_struct *mm,
+                                       enum page_walk_lock walk_lock)
+{
+       if (walk_lock == PGWALK_RDLOCK)
+               mmap_assert_locked(mm);
+       else
+               mmap_assert_write_locked(mm);
+}
+
+static inline void process_vma_walk_lock(struct vm_area_struct *vma,
+                                        enum page_walk_lock walk_lock)
+{
+#ifdef CONFIG_PER_VMA_LOCK
+       switch (walk_lock) {
+       case PGWALK_WRLOCK:
+               vma_start_write(vma);
+               break;
+       case PGWALK_WRLOCK_VERIFY:
+               vma_assert_write_locked(vma);
+               break;
+       case PGWALK_RDLOCK:
+               /* PGWALK_RDLOCK is handled by process_mm_walk_lock */
+               break;
+       }
+#endif
+}
+
 /**
  * walk_page_range - walk page table with caller specific callbacks
  * @mm:                mm_struct representing the target process of page table walk
@@ -459,7 +486,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
        if (!walk.mm)
                return -EINVAL;
 
-       mmap_assert_locked(walk.mm);
+       process_mm_walk_lock(walk.mm, ops->walk_lock);
 
        vma = find_vma(walk.mm, start);
        do {
@@ -474,6 +501,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
                        if (ops->pte_hole)
                                err = ops->pte_hole(start, next, -1, &walk);
                } else { /* inside vma */
+                       process_vma_walk_lock(vma, ops->walk_lock);
                        walk.vma = vma;
                        next = min(end, vma->vm_end);
                        vma = find_vma(mm, vma->vm_end);
@@ -549,7 +577,8 @@ int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start,
        if (start < vma->vm_start || end > vma->vm_end)
                return -EINVAL;
 
-       mmap_assert_locked(walk.mm);
+       process_mm_walk_lock(walk.mm, ops->walk_lock);
+       process_vma_walk_lock(vma, ops->walk_lock);
        return __walk_page_range(start, end, &walk);
 }
 
@@ -566,7 +595,8 @@ int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,
        if (!walk.mm)
                return -EINVAL;
 
-       mmap_assert_locked(walk.mm);
+       process_mm_walk_lock(walk.mm, ops->walk_lock);
+       process_vma_walk_lock(vma, ops->walk_lock);
        return __walk_page_range(vma->vm_start, vma->vm_end, &walk);
 }
 

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1080209..3555927 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4284,6 +4284,7 @@ static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_
        static const struct mm_walk_ops mm_walk_ops = {
                .test_walk = should_skip_vma,
                .p4d_entry = walk_pud_range,
+               .walk_lock = PGWALK_RDLOCK,
        };
 
        int err;